Semiconductor lasers with a high degree of spatial mode stability at high output power are of great importance for optical fiber systems applications. In particular, single-mode 980 nm semiconductor lasers are preferred pump sources for Er.sup.3+ doped silica fiber amplifiers as indicated by Okayasu et al. in Electronics Letters, Vol. 25, p. 1564 (1989) and Okayasu et al. in Photonics Technology Letters, Vol. 2, p. 689 (1990). The pump band at 980 nm is most efficient of the principle Er.sup.3+ absorption bands in terms of gain per input optical power. Pumping at 980 nm is also preferred in comparison to pumping at 1480 nm because less noise is introduced into the amplified optical signal.
In principle, the simplest way to achieve high power fundamental mode performance is to process the lasers as ridge waveguide structures. In practice, stringent dimensional tolerances are required to achieve good performance in these weakly-index-guided structures.
In such structures the geometry of the laser's lateral waveguide has to be carefully controlled to ensure that all of the power is in the fundamental spatial mode. A critical step in the fabrication of ridge waveguide lasers is control of the etch depth, which determines the lateral mode confinement.
If the ridge is not etched deeply enough, the laser is essentially gain-guided, and the high degree of anti-guiding from carrier-induced changes in refractive index in this system increases threshold current densities by more than an order of magnitude as reported by Shieh et al. in Appl. Phys. Letters, Vol. 54, p. 2521 (1989). If the ridge is etched too deeply, however, the laser will support multiple lateral (spatial) modes. Optimum performance requires that the distance from the bottom of the ridge to the active layer be controlled to better than 50 nm which, for these lasers, is typically less than 3% of the entire etch depth as shown by Crawford et al. in "Optical Amplifiers and Their Applications," paper WC4, Second Topical Meeting, Snowmass Village, Colo. (1991).
InP-based ridge waveguide lasers have been made using selective wet etchants and quaternary InGaAsP etch-stop layers. In contrast, GaAs-based ridge waveguide lasers have been made using timed etching because of the absence of a suitable etch-stop process. Crawford et al., supra, have previously reported InGaAs/GaAs Fabry-Perot lasers exhibiting single frequency operation to output powers as high as 170 mW. Those lasers, however, were fabricated by a technique that required tedious monitoring of the etch depth and exhibited non-uniform etch depth across the wafer.
In view of the poor control and reproducibility of the etch depth in fabricating GaAs-based heterostructures, the conventional technique of timed etching with no etch-stop layers is not a suitable procedure for producing GaAs-based ridge waveguide devices.